Photon-counting with single stoichiometric TiN layer-based optical MKIDs

We demonstrate the single photon counting mode at 405 and 850 nm with stoichiometric TiN-based microwave kinetic inductance detectors realized on a sapphire substrate and operated at bath temperatures over 300 mK. The detectors use single 15-25 nm-thick TiN layers featuring a critical temperature in the 2-3 K range. We found that the energy-resolving power R = E/Delta E exhibits an optimum with bath temperature, occurring in the 300-450 mK range, which can be almost double compared to those obtained at the lowest temperatures. Furthermore, the single photon regime is observed up to 700 mK. In addition to a high-temperature operation, the single stoichiometric layer would allow achieving a better uniformity in the critical temperature and, thus, kinetic inductance, compared to the often desired similar to 1 K sub-stoichiometric TiN.